1. Technical Field of the Invention
The present invention relates to ultraporous and microporous membranes which are useful in materials separations, by filtration, dialysis, and the like, and as supports and containment media for materials, and related uses. In particular, it relates to highly asymmetric, integral membranes with a skin and a porous sub-structure or support region.
2. Description of the Prior Art
A wide diversity of polymer membranes are known, and have attained wide applicability in diverse uses. Such membranes are characterized by a variety of properties and characteristics, and the selection of a membranes for a particular use is generally a function of the properties required or desired.
The most characteristic property of concern for most applications is the effective controlling pore diameter, which defines what materials may pass through the membrane, and which are retained. Ultraporous membranes are generally those with an effective controlling pore diameter of less than about 0.050 micrometers (or sometimes considered to be less than about 0.025 micrometers), down to as 0.005 micrometers, which is the realm of the size of a molecule of, for example, simple sugars and the like. Microporous membranes are those with effective limiting pore diameters of greater than ultraporous, normally greater than about 0.050 micrometers, up to about 1 micrometer, or occasionally more.
As used in the present application, the term "pore diameter" is employed to represent the span across skin pores or controlling pores of a membrane. It is not intended to suggest that all pores are circular and, indeed, most are not, as those of ordinary skill in the art will understand and as FIG. 2 illustrates.
Smaller pore membranes extend into the reverse osmosis region and below that into the gas separation region. Reverse osmosis membranes are used for ionic separations, under high applied pressure differentials, sufficient to overcome osmotic pressure, and are sometimes said to be dependent on a mechanism which is often characterized as intermolecular dissolution. Such membranes have a dense, non-porous surface skin, and do not function by effects dependent on seive-like characteristics. As a distinguishing characteristic, reverse osmosis is, in material part, dependent on the osmolarity of a solution as a determinant of the separatory characteristics of the reverse osmosis operation, while ultraporous and microporous membranes pass or retain materials predominantly on the basis of their size, at applied pressure differential which are commonly far less, often an order of magnitude less, than reverse osmosis operations, and are ordinarily considered to be substantially different in kind. Gas separation membranes operate on a molecular scale and fractionate gas mixtures based on size and absorption/desorption characteristics.
An important property of porous membranes is their permeability to flow. In the majority of applications, it is commonly desirable to process effectively the greatest volume of a feed material in the least amount of time. All other things being equal, the higher the flow rate of filtrate or related materials through the membrane, the higher the efficiency and economy of the procedure.
It has long been known that flow rates are proportional to pore diameters, and pore population. Taken together, these define an effective area through which fluids may pass. In practice, the relationship is ordinarily very approximate and highly variable.
Membranes may have a skin or may be skinless, i.e. with an isotropic structure from one face to the other. If a cast liquid film of adequate polymer concentration is quenched in a strong non-solvent, as with polysulfone solutions (or dispersions) quenched in water, the result is a "skinned" membrane, i.e., one with considerably smaller pores on the "skin" side than on the opposite side. If the quench liquid is a weak non-solvent, e.g., by adding solvent to the water, a more open skin and ultimately a skinless membrane can be produced.
When a skin is present, as generally in the case with gas separation, reverse osmosis, and ultrafiltration membranes and sometimes with microporous membranes, it is most often a dense film of polymer material with very small pores that extend into a support region of larger pores. If the pores are large enough, they can be observed by electron scanning microscopy, and this is true in the microporous range. However, because of the limitations of SEM techniques pores may not always be directly observable at diameters of less than about 0.050 micrometers, but their presence can be confirmed by the retentivity characteristics of the membrane.
Membranes can have different structures, generally determined by the technique by which it is synthesized. Examples include fibrous, granular, cellular, and spinodal, and they may be symmetrical, or asymmetric, isotropic or anisotropic (i.e., graded pore density).
Fibrous microstructure is most commonly associated with biaxial stretching of films of polymers. This is commonly employed, for example, in the production of porous membranes of polytetrafluoroethylene (TEFLON).RTM., in the microporous membranes commercially available as (GORETEX).RTM., among others. It is inherent in the nature of the process that the result is a skinless, symmetrical membrane.
Granular microstructure can be characteristic of membranes formed by the precipitation of polymer from certain formulations by a nucleation and growth mechanism. Globules or granules of precipitated polymer form and grow, and fuse with other such globules at their points of contact, leaving voids in the interstices which contribute the porosity of the granular mass. Such structures frequently contain "macrovoids" or "finger voids" in regions adjacent to skin imperfections which allow the quench liquid to penetrate the interior. The voids consequently are also skinned and lead to reduced membrane permeability. This occurs most commonly in ultraporous and reverse osmosis membranes. The techniques for the formation of such membranes are illustrated by Michaels, U.S. Pat. No. 3,615,024. The granular microstructure and characteristic "macrovoids" are illustrated by the photomicrographs shown in Wang, U.S. Pat. No. 3,988,245.
Cellular pore structures which are honeycombed or spongelike in appearance, are dependent presumably on a precipitation rate that is slower than with granular structures containing macrovoids. They can be skinned or unskinned. The latter structure generally is formed when the precipitation agent is moisture in the air (no liquid quenching during the curing process). A network of thin struts creates the system of contiguous polyhedral shaped cells. Liquid quenched membranes of this type are often associated with a dense or ultraporous skin.
Spinodal microstructure, as mentioned earlier, occurs when the polymer is precipitated by a spinodal decomposition mechanism, characterized by the formation of two separate liquid phases, one polymer rich and the other polymer poor, under conditions wherein each phase is continuous and dispersed in a characteristic pattern at the point at which the polymer precipitation occurs. Depending on the specific characteristics of the technique for attaining the spinodal decomposition mechanism, the resulting membrane may be, on the one hand, skinless, symmetrical, and uniform throughout, or skinned, asymmetric, and non-isotropic.
In the present application, the term spinodal structure is intended to mean the characteristic structure attained when a membrane is precipitated by spinodal decomposition, and to reflect the features illustrated in FIG. 1 and, in different scale, FIG. 8 which illustrate, by SEM photomicroscopy the remaining structure when the two, intertangled and intermixed continuous phases of spinodal decomposition are achieved. As those of ordinary skill in the art will understand, the spinodal structure represents one of the two continuous phases formed by the precipitated polymer, the other being the void volume within the structure.
The skinless symmetrical variety may be formed by thermal quench techniques or by solvent evaporation techniques. Thermal quenching techniques are illustrated by Castro, U.S. Pat. No. 4,247,498.
Skinned membranes with a highly asymmetric support structure are shown in Wrasidlo, U.S. Pat. No. 4,629,563, and Wrasidlo, U.S. 4,774,039. These membranes are formed by spinodal decomposition induced by solvent extraction from a cast metastable dispersion of two liquid phases, one polymer rich and the other polymer poor, in a liquid quench bath.
All the various techniques involved, and the membranes produced, have achieved a measure of commercial success. The spinodal microstructure, however, has often been preferred in a number of applications. As a general rule, the structure affords good mechanical properties, including tensile strength, elongation at break, and the like, the lowest hydraulic resistance to flow of any of the known microstructures, and offers opportunities to take advantage of the internal structure of the support as a depth filter, as a containment medium for materials, and other like advantages. As is well known to the art, the skinless, symmetric varieties have rather different uses that the skinned, highly asymmetric membranes of Wrasidlo.
In the dispersion casting technique of Wrasidlo, a number of disadvantages have been encountered. These include the following:
When the polymer is precipitated from the dispersion, there are frequent occurrences of small discontinuities. The reason for this is not fully understood, but the result is the formation, within the microstructure of the membrane support, substantial number, and at time vast numbers of tiny polymer spheres. These discrete spheres are difficult to remove by washing, and substantial numbers may remain in the membrane. This is highly undesirable, in most uses of the membranes, since there are few applications where the introduction of these spheres into a filtrate is acceptable. See the spheres illustrated in FIG. 7, which represent a severe case, after normal washing of the membrane.
The procedure for the formation of the membranes taught by Wrasidlo has over time proved to be excessively variable in the controlling pore diameter, flow rate for a given pore diameter, and in the occurrence of macro flaws in the integrity of the skin, leading to the loss of an unacceptable proportion of the membranes to a failure to satisfy necessary quality control standards. Quality control rejection of such membranes often has been substantial.
Some physical properties, including tensile strength and elongation at break, are often lower than desirable, and lower than required for the integrity of some otherwise desirable uses of these membranes.
These membranes are often employed in critical applications in the electronics industry, food processing, processing of biological materials, as sterilizing filters, and the like. Deficiencies in meeting the quality control requirements of such sensitive fields of use are quite unacceptable.